diff --git a/src/Illuminate/Routing/Middleware/ThrottleRequests.php b/src/Illuminate/Routing/Middleware/ThrottleRequests.php
index 894e9eaa53d0..8cfc54c374ff 100644
--- a/src/Illuminate/Routing/Middleware/ThrottleRequests.php
+++ b/src/Illuminate/Routing/Middleware/ThrottleRequests.php
@@ -119,7 +119,7 @@ protected function addHeaders(Response $response, $maxAttempts, $remainingAttemp
      */
     protected function calculateRemainingAttempts($key, $maxAttempts, $retryAfter = null)
     {
-        if ($retryAfter) {
+        if (! is_null($retryAfter)) {
             return 0;
         }